With a pair of rafters pushed apart to straighten the sagged roof a new mid span cross tie will be nailed in place.
Fixing sagging roof over garage.
The first step is to the basement first.
If you notice that there is a visible dip in your roof chances are you have a problem with one or several rafters and you may soon be facing more issues like water leaks bowed out walls or your roof caving in on you.
I prefer the term mid span cross tie to collar tie because i associate collar ties with placement high closer to the ridge.
If necessary use something to force the roof up while you work the beams into place and fasten them.
If the line does not continually follow the bottom of the beam and the bottom of the beam is below the line the beam is sagging.
Use instructions from my page how to fix a sagging floor before you continue trying to fix the roof sagging issue.
Any sagging will gradually be lifted and by the time you have finished the garage roof repair your roof should but upright again.
Sagging rafters can lead to serious problems with the entire roofing system.
Gradually work along all the beams pushing the roof up as you go.